This invention expands on the theory of active structural acoustic control as in U.S. Pat. No. 4,715,559 to Fuller. The Fuller patent teaches the art of controlling sound by controlling the efficiently radiating modes of a structure. Additionally, the theory of utilizing PVDF sensors is used in the invention.
Previous attempts at controlling large sound fields exist in many variations. Attempts by Jessel ("Secondary sources and their energy transfer," Acoustics Letters 4 (1981) 174-179) using control surfaces defined by planar arrays of microphones and speakers show an attempt at control from a non-compact source. Additionally, Davidson, Jr. et al. (U.S. Pat. No. 4,025,724) teach a method by which noise from non-compact sources can be controlled using a planar array of acoustic projectors and sensors.
A specific problem of a non-compact noise source which various people have tried to address is controlling the sound of a power generation transformer. This problem represents a non-compact noise source, and thus is useful to evaluate previous methods of non-compact noise source control. The creation of a sound barrier around a transformer is by no means unique. The principles described can be used in relation to the control of sound from other non-compact sources.
The control of transformer radiated noise is a problem whose satisfactory solution still remains to be found. Larger transformers consist of various configurations of metallic laminated cores and electrical windings immersed in an oil bath. The oil volume is usually contained in tank designed as a rectangular-like outer enclosure. Due to the magnetostrictive nature of the electrical excitation the excitation of the core appears as a sinusoid at twice the mains frequency plus harmonics. The winding and core are excited by the fluctuating magnetic force. These excite the oil field which in turn excites the outer casing. The outer casing then radiates sound. Due to the nature of the excitation the noise field is generally very tonal with peaks at the fundamental (twice the mains frequency) and harmonics. The noise fundamental is fairly low in frequency being around 100 Hz, and is thus difficult to control by passive means such as damping, stiffeners, etc. Furthermore, due to its long wavelength (of the order of 3.3 meters) the noise tends to diffract around barriers (such as beams, shields, etc.) located to control the sound.
Possibly one of the earliest attempts to actively control sound from transformers was described by Conover in Noise Control, Vol. 92, pp 78-82, "Fighting Noise with Noise", 1956. Conover experimentally investigated the use of acoustic sources arranged around a 150 MVA transformer close to its surface. The active acoustic sources in this case consisted of large loud speakers whose input was a control signal with adjustable amplitude and phase. Conover demonstrated that large attenuations of radiated sound could be achieved in the far-field. However, the attenuations were limited to selected angles and at other angles the sound was increased in magnitude. This result is undoubtedly due to the large size of the transformer relative to wavelength of the sound. The transformer cannot be considered as a compact source when its characteristic dimensions are greater than an acoustic wavelength, and thus its noise field cannot be globally controlled with a low number of control acoustic sources.
The next interesting work was carried out by Hesselman who looked at active control of sound radiation from a far smaller, 100 kVA transformer. In this arrangement two loudspeakers were used located at either end of the transformer. The residual or controlled noise field had the characteristic of a longitudinal quadrapole which has a very low radiation efficiency at low frequencies. Hesselman also employed a control system for the first time that was essentially feed-forward. The second harmonic of the mains signal was used to trigger a signal generator. The output of the signal generator was passed through a multi-channel phase shifter and amplifier and then to the compensation (active) acoustic sources. The amplitude and phases of the compensation signal were adjusted so as to provide a control field very close to the noise field at the measurement points in the far-field. Once adjusted the phase to the compensation speakers was flipped through 180 degrees and the residual field measured. All experiments were performed in an anechoic chamber. It should also be noted that the noise field was dominated by the fundamental by 20 dB over the harmonics.
These results demonstrate global attenuation of the order of 20-40 dB depending upon observation angle. An additional interesting result was that the sound levels rose in the transformer near-field while they were attenuated in the far-field.
As discussed by Hesselman the global control exhibited in his tests are due to the small size of the transformer (approximately 2 m.times.1 m.times.1 m) relative to the wavelength (approximately 3.3 m). This is apparent in the noise field of the transformer studied by Hesselman which exhibits the omni-directional, monopole directivity radiation pattern associated with a compact source unlike the case studied by Conover. Hesselman also points out that in the application of the active technique to a large transformer, the noise source can be considered as being composed of a number of locally compact sources whose linear dimensions do not exceed one third of a wavelength. Each of these sub-sources can then be thought of as a compact or monopole source of a particular source strength and phase. This type of arrangement may be then controlled by the use of a set of active acoustic sources, independently controlled, positioned over and very near the center of each sub-panel. The active sources would have opposite phase and the same source strength as their associated sub-panel. Hesselman thus foreshadowed the use of "arrays" of acoustic sources as used by his later counterparts.
Experiments of the use of "arrays" were carried out by Angevine who described his work in Proceedings of Inter-Noise 81, pp 303-306, "Active Acoustic Attenuation of Electronic Transformer Noise", 1978. Angevine studied active control of transformer noise using arrays of sound sources arranged around the transformer. His results generally support what is stated above. If the transformer physical size is large compared to the acoustic wavelength then arrays of many acoustic sources arranged around the transformer will be needed to proved global control. Otherwise attenuation will be achieved at selected radiation angles towards error microphones but increase towards other radiation angles (control spillover).
The work of Ross is described in Journal of Sound and Vibration, Vol. 61(4), pp 473-476, "Experiments on the Active Control of Transformer Noise", 1978. Ross's work investigated active control of transformer radiated noise in a realistic application. In this situation two noisy transformers were located across a courtyard from offices in which the transformer noise was extremely annoying. The active control was realized by using a loudspeaker located near the transformer. Investigation of the noise field showed that it was relatively uniform when it reached the offices suggesting that the noise source was acoustically compact. For the active compensation, sound was picked up by a detector microphone and fed through a set of filter networks corresponding to the fundamental and first two harmonics (100, 200, 300 Hz). The output of these phase and amplitude controlled signals were then summed and fed into the active loudspeaker. With the loudspeaker in a variety of positions the system phases and amplitude was adjusted to minimize the noise at a number of positions in the offices.
The results showed that for the lowest frequency of 100 Hz the sound was reasonably globally controlled by between 10 to 28 dB throughout the office room. The higher frequencies of 200 and 300 Hz could only be controlled locally in areas of approximately 1 meter radius around the error microphones. Ross concludes, as with the previous work, that by "using more loudspeakers the control could be greatly improved."
The work of Eatwell is described in the Proceeding of the Institute of Acoustics, 9(7) pp. 269-274, "The Active Control of Transformer Noise,"1987. This work describes the results of computer optimizations for the positions of the control actuators for a 0.5 MVA transformer. The results demonstrate that the number of actuators required is proportional to the square of the frequency to be controlled.
The above work can be summarized as follows. When the transformer is compact relative to the wavelength of the noise then a low number of active acoustic sources will be required. A compact source is usually indicated by a relatively uniform radiation field with angle, around the transformer. When the transformer's dimensions are of the order of the wavelength, the radiation field exhibits complex lobes and arrays of acoustic sources arranged around the transformer at the center of areas of approximately lamda/3.times.lamda/3 in size will be needed where lamda is the acoustic wavelength. Systems such as this can be implemented, however, there are a number of practical disadvantages, amongst which are the high number of control channels needed. However, it is probably the sheer size and bulkiness of the active acoustic sources arranged around the transformer that has prevented their use. It is in this sense that the active acoustic panel solves the problem. In summary, the active panel provides a compact method to introduce the degrees of freedom necessary to control the non-compact acoustic source.